This invention relates generally to digital telephone exchanges and more particularly to a digital telephone exchange which simultaneously interfaces with telephone networks having various PCM circuits of differing bit rates and protocols and provides compatible connections between them. This invention is related to the invention described in U.S. Pat. No. 4,860,281, "Individual Subchannel Loopback in the PCM Interfaces of a Digital Telephone Exchange With Control of the Outbound Path" filed on behalf of Finley et al.
Many digital exchanges interface the central network through which calls are routed (commonly called the switch) to the outside world, all or in part, via Time-Division-Multiplexed (TDM) Pulse Code Modulated (PCM) four-wire circuits which can carry 24 or 30 or more subchannels. These circuits are often called "PCM groups." Each subchannel is allocated specific bit positions for speech or data and for channel associated line signaling for either trunk use or subscriber line use or for unrelated use. Where the trunks employ common channel signaling, the line signaling bit positions may be used for that purpose of signalling, or even go unused as might also be the case in certain data and audio applications.
PCM got an earlier start in North America than in Europe and has evolved based primarily on a 24 subchannel first level multiplexer, transmitted at 1.544 Mbps, with several speech coding formats, line signal formats and transmission formats. Today, the analog speech is all coded into digital representations using an algorithm known as .mu.-Law. In the 24 channel formats known as DS-1 and the Extended Superframe Format (ESF) in general use today, line signaling generally shares speech-bit positions but formats are still evolving, and other 1.544 Mbps formats with separated speech and signaling will ultimately replace DS-1 and ESF. Higher level multiplexers combine multiples of DS-1/ESF groups into 48, 96, 672, 4032 and various other numbers of subchannels depending upon the application.
The rest of the world has generally followed CCITT format recommendations. Starting later, and learning from the North American DS-1 experience and utilizing later technology, the CCITT format calls for a first level multiplexer of 30 subchannels (commonly called CEPT-30 or simply CEPT) with independent line signaling fields and with A-Law speech coding, transmitted at 2.048 Mbps. Higher level multiplexers are mostly 4:1 concentrators, such as 120, 480 and 1920 subchannels. Notwithstanding the attempts at standardization, different telephone administrations employ different impedances, different transmission media (i.e., coaxial cable or twisted pairs), different transmission protocols, different line signaling conditions, different interfacing levels and so forth.
Most digital exchanges are designed to accommodate one basic first level multiplex PCM bitstream rate and protocol. Some can be adapted to interface either of several rates, but only in entirety. None are known to be able to accommodate odd protocol and data conversions on a per-subchannel basis. Thus, if by external applique one or more DS-1/ESF groups are converted to or from one or more CEPT groups, all (or maybe none) of the subchannel speech/data will undergo .mu.-Law/A-Law conversion (a simple table look-up) in a manner that is beyond control of the exchanges in tandem with a particular connection. There are complications in CEPT/CEPT and DS-1/ESF/CEPT line signal conversions which cross national boundaries, CCITT standards notwithstanding.
With the advent of the Integrated Services Digital Network (ISDN), it will be desirable, but sometimes frustratingly difficult to achieve end-to-end subchannel paths across national boundaries that are free of extraneous conversions. There is at least one category of exchanges known generally as Digital Access and Cross-Connect Systems (DACCS) in which it would be possible for the customer (subscriber) to impose the level of microcontrol required to achieve the desired end result.